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Inclined Profiling of PRBs and Subsurface Barriers

GeoSierra has developed inclined profiling of PRBs and subsurface barriers for QA/QC design/build services.

The method removes the complexity of direct sampling and involves either a magnetometer or electrical resistivity probe.


Inclined Profiling

Verification procedures of the in situ constructed geometry of the permeable reactive barrier (PRB) are important for high confidence in the quality control and quality assurance of the constructed PRB. Difficulties in direct sampling of the iron PRBs have been experienced at both shallow and moderate depth due to the inability to contain the iron filings within the sampler tool. At significant depth and/or in flowing ground conditions, direct inclined samplings of undisturbed iron PRBs have been found to be extremely difficult. Under these conditions the ground in front of the sampler flows into the outer protective casing when the sampler is withdrawn and thus disturbs the ground for the next sampling interval. The next sampling attempt then samples disturbed ground consisting of a mixture of native soils and iron filings.

Such direct PRB sampling difficulties have led to the utilization of an inclined direct push soil electrical conductivity/resistivity probe to determine indirectly the iron PRB geometry. The electrical resistivity contrast between native soils and the iron PRB are generally sufficient to clearly identify the iron PRB geometry. The electrical conductivity probe is approximately 6" in length and 1.5" in diameter. The probe contains four electrodes spaced approximately 1" apart. The probe electrodes are excited by a low voltage current and soil conductivity can be quantified by the instrument. When the probe penetrates an iron PRB wall, a significant change in conductivity is recorded, thus providing a measurement of its thickness.

An alternative method is to incline drill through the PRB and install a temporary non-metallic casing across the PRB intersection. A borehole magnetometer probe is then run in and logs the thickness of the iron PRB directly. Since the magnetometer measures the presence of the iron directly, then this method is considered superior to the inclined electrical resistivity probe.


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